This invention relates to a method of manufacturing semiconductor devices which includes an improved element isolation step and semiconductor devices manufactured by the method.
The manufacture of a semiconductor device involves a step of forming an element isolation region constituted by an insulator for isolating a transistor region (or element region) of a semiconductor substrate as is well known in the art. Particularly, recent higher density integration of semiconductor devices is calling for the establishment of finer element isolation region formation techniques.
Hitherto, a selective oxidation method has been used to form an element isolation region constituted by an insulator. In this method, a thermal oxidation film 2, as shown in FIG. 1A, is grown to a thickness of 1,000 .ANG. on, for instance, a p-type silicon substrate 1 with plane index (100). A silicon nitride film 3 is then deposited to a thickness of, for instance, 2,000 .ANG. on the film 3. Subsequently, a resist film 4 covering an element region is formed on the silicon nitride film 3 by using the photoetching method. The silicon nitride film 3 is then etched with the resist film 4 used as a mask to form a silicon nitride film pattern 5 which serves as an oxidation-proof mask material. Afterwards, an element isolation layer field inversion prevention impurity (for instance boron) is ion implanted through the thermal oxidation film 2 into the substrate 1 with the resist film 4 used as a mask (as shown in FIG. 1B). The resist film 4 is then removed, and the wafer is thermally treated in a high temperature wet oxygen atmosphere with the silicon nitride film pattern 5 as a mask to form an isolation oxide film 6, for instance 1 .mu.m thick, and a p.sup.+ -type inversion prevention layer 7 thereunder (as shown in FIG. 1C). The silicon nitride film pattern 5 and thermal oxidation film 2 are removed afterwards (as shown in FIG. 1D).
In this prior art method, however, a so-called bird's beak 8, as shown in FIG. 1C, is formed as a result of intrusion of the element isolation oxide film 6 into the element region. The bird's beak increases the resist film pattern width; for example, if the bird's beak along one edge has a width dimension of 0.5 .mu.m, a design resist film pattern width of, for instance 2 .mu.m is increased to 3 .mu.m. This means a reduction of the substrate area that can be used as the element area and is hence undesired from the standpoint of the formation of precision element isolation region and higher integration density. In addition, the p.sup.+ -type inversion prevention layer 7 also intrudes into the element region as shown in FIG. 1D and has adverse effects upon a diffusion layer which may be formed as a surface layer in the element region. Further, nitrogen compounds are liable to be diffused from the silicon nitride film through the thermal oxidation film into the silicon substrate at the time of the thermal oxidation to result in the formation of a so called white ribbon layer on the silicon substrate.